1. Field of the Invention
The present invention relates to methods for forming electrodes of ceramic chip electronic components. In particular, the present invention relates to a method for plating electrodes of ceramic chip electronic components and to a ceramic chip electronic component having electrodes plated by the method.
2. Description of the Related Art
Electrodes of electronic components formed of ceramics, glass, plastics and the like, are generally plated with a tin-lead alloy in order to improve the solder wettability thereof. Tin has been replacing the tin-lead alloy used to plate the electrodes as the desire for lead-free plating has intensified. However, the intrinsic physical properties of tin cause various problems in plating.
In particular, when ceramic chip electronic components (chips) containing ceramic and glass is subjected to tin plating, the chips are liable to adhere to one another. This adhesion is caused by, for example, tin plating using a well-known weakly acidic plating bath containing stannous sulfate.
This adhesion means that, as shown in FIGS. 1A to 1C, tin-plated portions (tin-deposited portion) 5 of at least two chips 1 adhere to each other during tin plating. It is considered that the adhesion is caused by the hardness of the tin plating film, which is softer than the tin-lead plating film.
In particular, when chips are subjected to barrel plating, each of the chips is agitated differently in the plating barrel. For example, the chips are in contact with adjacent chips for a long time in a region where chips are hardly agitated. As a result, these adjacent chips adhere to one another.
This adhesion problem becomes particularly significant when the number of chips to be plated at a time is increased. If the adhesion occurs, adhering chips themselves are defective. In addition, some weakly adhering chips are likely to be separated during plating or drying. The thickness of the plating films at the separated portions of these chips is very thin and, accordingly, these chips are defective. Thus, defective chips are likely to be mixed among normal chips.
In particular, when two chips 1 adhering to each other, as shown in FIG. 1A, separate, the thickness at the portions (separated portions 5a) of the plating films which adhered to each other become much smaller, as shown in FIG. 2. The thickness of the separated portions 5a measured in practice was, for example, 0.1 μm, and this thickness is much smaller than the thickness of 5 μm of normal tin-plating films 5.
Mounting characteristics, such as solderability, of chips having such a very thin plating film are degraded. As chip components become smaller, it is expected that this adhesion problem will become more serious.
In order to prevent the adhesion problem, processing conditions of chips have been improved. For example, the number of chips to be plated at a time is reduced to reduce the adhesion. However, in order to prevent the degradation of working efficiency and to advance lead-free tin plating, a fundamental solution is desired.